A high-intensity discharge lamp is of compact size, offers a high luminous flux, and is analogous to a point source of light, and also light distribution is easy to control. Therefore, it has been widely used as an alternative to an incandescent lamp or a halogen lamp in recent years. For lighting this high-intensity discharge lamp, there is a discharge-lamp lighting device that switches DC voltage on and off at high frequency and converts it into a low-frequency square wave via an inductor and a capacitor thereby to operate the high-intensity discharge lamp. When high-frequency electric power is used to light the high-intensity discharge lamp, arc discharge becomes unstable due to acoustic resonance, and therefore the lamp flickers or goes out. In contrast, AC-powered lighting by the low-frequency square wave ensures stable arc discharge continuously.
Upon start-up of the high-intensity discharge lamp, it requires a dielectric breakdown caused by an application of impulse voltage of several to tens of kV. Further, following the occurrence of the dielectric breakdown between electrodes, a transition from glow discharge to arc discharge is required. Thus, energy needs to be supplied in optimum conditions depending on each operating state, and also control needs to be made to achieve a smooth transition from a start-up state to a stable lighting state.
Japanese Patent Application Laid-Open No. 2004-265707 has proposed that, upon start-up time of the high-intensity discharge lamp, a period A during which high voltage is applied by resonance operation and a period B during which low-frequency-square-wave voltage is applied, are alternately repeated during a period C, as shown in FIG. 7. The dielectric breakdown between electrodes is ensured by the period A for applying the high voltage by the resonance operation, and the transition from the glow discharge to the arc discharge is ensured by the period B for applying the low-frequency-square-wave voltage.
Recent studies have found that it is important to set the periods A, B, and C in FIG. 7 to their respective optimum lengths. Specifically, a problem has been found that, when the period A for applying the high voltage by the resonance operation is too short relative to the period B for applying the low-frequency-square-wave voltage, the transition from the glow discharge to the arc discharge can be made smoothly, but dielectric breakdown performance is deficient. In contrast, the period A for applying the high voltage by the resonance operation is too long relative to the period B for applying the low-frequency-square-wave voltage, the dielectric breakdown is possible, but the transition from the glow discharge to the arc discharge is not made smoothly.
The present invention has been made in consideration of the foregoing problem, and an object thereof is to provide a discharge-lamp lighting device and a luminaire that provide optimum dielectric breakdown performance and arc transition performance, which are two major factors in starting a high-intensity discharge lamp, by determining and defining optimum conditions from qualitative and quantitative points of view for a period during which high voltage is applied by resonance operation, a period during which low-frequency-square-wave voltage or DC voltage is applied, and a cycle of these repetitive periods, thereby to realize smooth start-up of the high-intensity discharge lamp.